Emotions Vented Online Are Contagious, Study Finds

By Robert Lee Hotz

In the digital swirl of Facebook status updates, emotions expressed online can be contagious, according to a new study encompassing more than 100 million people in the U.S. and a billion messages they posted.

Moreover, upbeat messages were far more likely than negative ones to affect the mood of others online, researchers at the University of California, San Diego, Yale University and Facebook Inc. reported Wednesday in one of the largest public studies of the social network to date.

To protect privacy and confidentiality, Facebook made the data anonymous. The researchers used an automated text-analysis system, so they didn’t see the actual content of messages.

“We wanted to see if emotional changes in one person caused emotional changes in another person and that’s exactly what we found,” said UC San Diego political scientist James Fowler, who was lead author on the research published online in the journal PLOS ONE.

First, the researchers found that a rainy day directly influenced the emotional tenor of a person’s Facebook posts. The effect was small but significant—the number of negative posts rose 1.16%, while the number of positive comments fell 1.19%.

That, in turn, affected the Facebook status of one or two friends in other cities where it wasn’t raining. Each additional positive post resulted in a further 1.75 positive posts among friends; while each negative post yielded 1.29 more negative posts by friends, the researchers said.

Posts were sorted by whether they contained positive or negative language, such as the word “sad” or “happy.” To strip out the effect of topic contagion, the researchers removed any status updates that were actually about the weather. “We wanted posts where it is raining on you and it is making you write negative posts that are not about the weather,” Dr. Fowler said.

With about 1.2 billion active users, a volatile upbeat or downbeat mood could quickly spread through Facebook, Dr. Fowler said. “It is going to have implications for financial markets, which have bubbles and busts, and it has implications for political activity,” he said.

In a 2012 study of social media and voting behavior, Dr. Fowler and his colleagues reported that a single Election Day message sent out across Facebook likely caused an additional 340,000 voters to turn out for the 2010 U.S. midterm elections.

For that experiment, Dr. Fowler and his colleagues sent out targeted messages to about 61 million people 18 or older who accessed their Facebook accounts on Nov. 2, 2010. They sent some people a simple informational message urging them to vote. They sent many others a “social” voting message that featured photos of Facebook friends who indicated they had already voted. A control group received no message at all.

Electoral records later indicated that those who saw the message featuring their friends were slightly more likely to have voted, Dr. Fowler reported in Nature.

In previous research, Dr. Fowler and Yale sociologist Nicholas Christakis—a co-author on the new study—popularized the idea that obesity, smoking, emotions and ideas can spread through networks of friends and acquaintances. They wrote a 2009 book on the subject called “Connected.” Those earlier studies have prompted skepticism from some scholars who questioned the statistical methods used.

A debate over the true impact of online behavior has raged since the birth of social media, but sociologists and psychologists rarely have access to enough raw data from personal interactions that would reveal whether online feelings could influence people in the same way as in face-to-face encounters. Social-media companies have been reluctant to share users’ private information.

In the new study, Dr. Fowler and his colleagues studied status updates posted by 100 million users in the 100 most populous U.S. cities between January 2009 and March 2012. To further protect confidentiality, the data were kept on Facebook’s secure servers and handled by the company’s data scientists, two of whom were co-authors on the research paper.